Typical commercial polymers, particularly the rubbery, thermoplastic polymers such as polyisobutylene, polybutadienes and the like, encompass a fairly broad molecular weight range. A number of different techniques have been developed to fractionate such polymers according to molecular weight. Recognized methods for polymer fractionation include fractionation by solubility differences; by chromatography; by sedimentation; by diffusion; by ultrafiltration through porous membranes; and by zone melting. All of these methods are restricted in practical use because they are slow, expensive and difficult if not impossible to scale up to any reasonably large production rate.
Perhaps because of the difficulty and expense of obtaining high molecular weight fractions of common polymers, no significant commercial market for these polymer fractions has developed. However, it is now recognized that high molecular weight fractions of certain polymers provide unique advantages in specific applications. For example, the polymer weight concentration in a hydrocarbon required to enhance the rate of fluid flow through a conduit or to impart antimisting properties to jet fuels, kerosene and diesel fuels is an inverse and exponential function of molecular weight. In the case of polyisobutylene used as an antimisting additive to kerosene, it has been empirically determined that less than 50 ppm of a 5 million molecular weight polymer provides an antimisting effectiveness equal to about 1000 ppm of a 3 million molecular weight fraction.
Focusing now on those fractionation methods which utilize solubility differences to achieve separation, all of those methods first require that the polymer be dissolved in a suitable solvent. The solution process especially for relatively high molecular weight polymers, is very slow, often taking days to weeks. The polymer when immersed in a solvent first swells to a gel-like state and then slowly disperses to form a solution. The rate of solution can be speeded by heating, agitation, or both but at the cost of polymer degradation and a reduction of the average molecular weight. Hence, if the object of the fractionation process is to recover a high molecular weight fraction, all attempts to speed dissolution reduce the quality of the desired fraction.
After a polymer solution has been obtained, fractions of differing molecular weights can be separated from the solution in several different ways. One approach to separating a high molecular weight fraction is to add to the polymer solution a miscible non-solvent, typically acetone. The non-solvent causes precipitation of a solvent-rich polymer gel which may be removed by decantation. Successive additions of non-solvent, if properly controlled, results in the successive precipitation of polymer fractions of progressively decreasing molecular weight. Another method of separating polymer fractions is to add a quantity of non-solvent to the polymer solution and to thereafter obtain successive fractions of decreasing molecular weight by evaporating off portions of the more volatile solvent. Yet another approach is to essentially saturate a solution with polymer at a moderately elevated temperature and to thereafter cool the solution thereby precipitating out a high molecular weight fraction of the dissolved polymer. A small quantity of non-solvent may be added here as well to improve the fractionation obtained.
Traditional methods of dissolving polymers in solvents, as noted previously, usually require an extended time period for completion. There has recently been developed a technique for the very rapid dissolution of polymers in compatible liquids. This technique, disclosed and claimed in U.S. Pat. No. 4,340,076, the disclosure of which is incorporated by reference herein, comprises the cryogenic comminution of polymers to form activated particulates which are thereafter admixed with a suitable liquid solvent. The active polymer particles dissolve very rapidly, essentially instantaneously, in the solvent liquid with minimal degradation of the polymer or reduction of its average molecular weight.